Here's a Catholic answer from the internet:
The term Mass comes from the Latin word missa meaning dismissal. The catechism now defines Mass as, “The Eucharist or principal sacramental celebration of the Church, established by Jesus at the Last Supper, in which the mystery of our salvation through participation in the sacrificial death and glorious resurrection of Christ is renewed and accomplished. The Mass renews the paschal sacrifice of Christ as the sacrifice offered by the Church. It is called "Mass" (from the Latin missa) because of the "mission" or "sending" with which the liturgical celebration concludes.”
The term "Mass" may be derived from the Late Latin word missa (dismissal), a word used in the concluding formula of Mass in Latin: "Ite, missa est" ("Go; it is the dismissal")